A modern industrial control and monitoring system typically includes one or more process controllers capable of controlling one or more process control elements. The process controller is typically implemented as one or more control modules, responsible for different aspects of the process control function. For example, a control module may control the pressure of a process with a data acquisition block that senses pressure and provides the measured pressure as an input to a control block executing a proportional-integral-derivative (PID) algorithm. The PID block may, in turn, generate output signals to control a valve actuator output block. The same control module may also have data acquisition/PID/output blocks sensing and controlling the temperature of the process.
An operator of a control system may save the current static control module or block configuration of the process controller in a snapshot file. When the configuration of the controller is changed, the snapshot file may be updated with the new configuration. If control modules or blocks of the control system subsequently fail or must otherwise be replaced, the previous configuration of the controller may be restored by loading configuration data from the snapshot file. However, in what is known as a “cold restart,” the controller will initially be executing in manual mode. The operator will be required to reset operational parameters to return the controller to a fully operational state.
A checkpoint file, in contrast, contains data describing the static control module and block configuration of the control system as well as data describing the dynamic operational parameters of the control modules and blocks of the control system. This information may be temperature or pressure set points, alarm points, gain factors, or other such operational parameters of the process controller. When replacing or restarting a module or device, loading data from a checkpoint file allows not only the configuration of the controller to be restored, but also its operating characteristics. This allows the controller to be returned to a fully operational state, in what is known as a “warm restart.”
However, there are structural elements of the process controller, such as graphical presentation elements, that are neither saved in, nor restored from, a checkpoint file. For example, the operator of an is industrial control system typically has a user interface presenting a view of several process controllers in the system. This user interface allows the operator to monitor the operation of the system. When only one process controller must be restored and restarted, its checkpoint file does not affect the monitor view presented to the operator.
Typically, when the configuration of a process controller is changed or loaded, for example by moving data acquisition/PID/output temperature control blocks from one control module to another, the monitor user interface view and other structural elements are also changed. This provides the operator with a consistent view of the control functions actually being performed by the process controller. However, if, after such a change to the configuration and structure has been made, the process controller is restarted by restoring it from a checkpoint file made before the change, problems may be created such as so-called “ghost points” or unresolved control connections.
An example of a “ghost point” is a restored control block in one control module of the process controller whose operation the operator can no longer monitor in the user interface. This is because the user interface structure indicates that the control block is now in another control module. An unresolved control connection occurs where a control block previously had a connection to another upstream or downstream control block, however that other control block no longer exists in the changed structure of the control system. In this situation, the operator's view of the connections between control blocks in the process controller is no longer consistent with the actual connections between control blocks.